mm/page_table_check: fix crash on ZONE_DEVICE

Not all pages may apply to pgtable check.  One example is ZONE_DEVICE
pages: they map PFNs directly, and they don't allocate page_ext at all
even if there's struct page around.  One may reference
devm_memremap_pages().

When both ZONE_DEVICE and page-table-check enabled, then try to map some
dax memories, one can trigger kernel bug constantly now when the kernel
was trying to inject some pfn maps on the dax device:

 kernel BUG at mm/page_table_check.c:55!

While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page
fault resolutions, skip all the checks if page_ext doesn't even exist in
pgtable checker, which applies to ZONE_DEVICE but maybe more.

Link: https://lkml.kernel.org/r/20240605212146.994486-1-peterx@redhat.com
Fixes: df4e817b7108 ("mm: page table check")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 4169576bed72985ef7e18af15254dfcfb5fc95f4..509c6ef8de400e78bb247deaf4e7e2b6a79ff357 100644 (file)
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -73,6 +73,9 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
        page = pfn_to_page(pfn);
        page_ext = page_ext_get(page);
 
+       if (!page_ext)
+               return;
+
        BUG_ON(PageSlab(page));
        anon = PageAnon(page);
 
@@ -110,6 +113,9 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
        page = pfn_to_page(pfn);
        page_ext = page_ext_get(page);
 
+       if (!page_ext)
+               return;
+
        BUG_ON(PageSlab(page));
        anon = PageAnon(page);
 
@@ -140,7 +146,10 @@ void __page_table_check_zero(struct page *page, unsigned int order)
        BUG_ON(PageSlab(page));
 
        page_ext = page_ext_get(page);
-       BUG_ON(!page_ext);
+
+       if (!page_ext)
+               return;
+
        for (i = 0; i < (1ul << order); i++) {
                struct page_table_check *ptc = get_page_table_check(page_ext);